A certain number of refining and petrochemical methods rely on reactors that are called radial reactors. Among these methods, it is possible to cite—without being limiting—the catalytic reforming of gasolines, oligocracking of olefinic cuts, dehydration of alcohols (ethanol, propanol, isobutanol), skeletal isomerization of olefins, metathesis for the production of propylene, dehydrogenation of paraffins.
In a radial-bed reactor, the catalytic bed has the shape of a vertical cylindrical annulus that on the interior side is bounded by an interior screen, called screen of the central collector, retaining the catalyst, and on the exterior side, either by another screen of the same type as the interior screen and called outer screen, or by a device consisting of an assembly of scallop-shaped screen elements (called “scallops” in English terminology).
This reactor geometry is that which is used predominantly in the industry, but it necessitates an outer screen/minimum central collector space to ensure that, during maintenance operations on the reactors, the operators can go between the screens so as to inspect them and to clean them (fragments of catalysts deposited between the screens).
The current sizing regulations call for two criteria that can prove contradictory for radial beds: a minimum catalytic bed thickness of about 400 mm so as to achieve the minimum boundary for the maintenance of the screens, and a minimum pressure drop in going through the bed of between 20 and 80 mbars (mbar is the abbreviation for millibar or 10−3 bar) in the interior of the radial bed to maintain a proper distribution of the gas in the catalytic bed, without blocking the flow of the catalyst.
Now, for the reduced capacities of reactors, maintaining a sufficient pressure drop for proper distribution of the feedstock over the entire height of the reactor implies shortening the bed (increasing superficial velocities).
Furthermore, below a certain critical capacity, this shortening is limited by the minimum thickness of 400 mm, not making it possible to maintain the pressure drop criterion. Therefore, a need exists for a technological solution that makes it possible to be free of at least one of the two criteria, optionally both, so as to maintain the possibility of sizing the radial beds, in particular moving beds, for the reduced capacities.
In the prior art relating to radial-bed reactors, it is possible to cite the U.S. Pat. No. 6,221,320, which provides a rather complete summary of the conventional technologies.
According to the state of the art, the catalytic bed in a moving-bed radial reactor is defined by two screens, an inner screen and an outer screen. More specifically, a distinction is generally made between:                An inner screen that defines the central collector of the gaseous effluents,        An outer screen that defines the space for supplying the feedstock in the gaseous state.        
The processed fluid arrives by the outer space that is defined between the shell ring and the outer screen. It then passes through the catalytic bed in a manner that is approximately horizontal and perpendicular to the circulation of the catalyst that is gravitational, i.e., approximately vertical from top to bottom, and is obtained as a result of the weight alone of the catalyst bed.
The processed fluid in radial flow and the catalyst in gravitational flow are separated by the inner screen that generally has a cylindrical shape, with the same approximately vertical axis as the outer screen.
The cylinder, or more generally the approximately cylindrical shape, defined by the inner screen, serves as central collector to drain the gaseous effluents from the reaction zone that is between the outer screen and the inner screen and therefore of approximately annular shape.
The constraints linked to the moving-bed radial technology are numerous. In particular, the gas velocities in going through the catalytic bed are limited in order to:                avoid cavitation on input to the bed,        avoid blocking of the catalyst at its exit against the inner screen,        reduce the pressure drops based on the velocity and the thickness of the bed.        
For reasons of uniform distribution over the entire height of the catalytic bed, a perforated tube designed to create the pressure drop can be added onto the central collector.
Finally, for construction reasons, it is often necessary to leave a sufficient space between the inner screen and the outer screen. Ultimately, when all of the constraints for this reactor configuration are accumulated, the minimum volume of catalyst that can be enclosed in the annular zone cannot fall below a certain minimum value.
Generally, according to the prior art, the maximum accessible PPH in radial moving beds are on the order of 20 h−1, whereas the reactor according to this invention makes it possible to reach PPH of higher than 50 h−1, even higher than 100 h−1.